1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine rotor blade.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine with multiple rows or stages of rotor blades and stator vanes that are exposed to a hot gas flow to convert the energy of the gas flow into mechanical energy. It is well known that the turbine efficiency can be increased by passing a higher temperature gas flow into the turbine. The turbine inlet temperature is limited to the material properties of the turbine, especially of the first stage vanes and blades, and to an amount of cooling of these airfoils. Better cooling capability would keep the metal temperature of the airfoils relatively low enough to allow for higher temperature gas flow. Complex cooling circuits have been proposed that include combinations of impingement cooling and convection cooling of the internal metal, and then film cooling on the outer airfoil surface. Of these types of cooling, impingement cooling offers the best heat transfer coefficient.
Another problem with turbine airfoils is maintaining a proper metal temperature of each part of the airfoil. Some surfaces are exposed to higher gas flow temperatures and thus can result in a hot spot on the airfoil. Hot spots can cause early erosion damage that will limit the life period of the airfoil. Especially in an industrial gas turbine engine, part life is an important design criterion since these engines operate on a continuous time period of 48,000 hours without shut down. If a part is worn or damaged, the efficiency of the turbine can be significantly affected. Therefore, cooling of specific parts of the airfoil must also be considered and provided for.
Still another design issue involves the cooling air pressure so that back flow margin (BFM) does not cause problems. BFM is when the external hot gas pressure is greater than the cooling air pressure for a film cooling hole. This situation will result in the hot gas flowing into the airfoil through the film cooling holes. Therefore, the cooling circuit must be tailored for the local pressure distribution to optimize the film cooling. Too little film cooling discharge would result in low cooling protection, while too much film cooling discharge would result in wasted cooling air which also decreases the engine efficiency.